Songbirds represent a well-established model to study vocal communication and learning and the effects of sex steroids on brain and behavior. Intriguingly, the neuronal circuitry involved in the perception, discrimination and memorization of song also expresses high levels of aromatase, an enzyme involved in the synthesis of estrogen from its precursor testosterone. This overlap between song-processing and estrogen-generating elements occurs within the caudomedial nidopallium (NCM), a central auditory area comparable to a portion of the auditory cortex of mammals. The functional consequences of the enriched expression of aromatase in NCM are unknown. The experiments in this R21 proposal are highly exploratory in nature and are designed to address two hypotheses regarding the potential role of aromatase expression in NCM: 1) We hypothesize that estrogen, both circulating, and produced locally in NCM through the action of aromatase, modulates the responsiveness of song-processing circuitry, thereby regulating the bird's response to auditory cues important for territorial and reproductive behaviors. To test this hypothesis, we will examine whether the expression levels of known and novel song-induced genes in NCM are modulated by estrogen. 2) We hypothesize that the activation of song-responsive neuronal circuitry in NCM regulates the local production of estrogen, which could have important consequences for brain physiology and behavior. To test this hypothesis, we will conduct direct measurements of aromatase activity and estrogen levels in the NCM of song-stimulated birds. Although much research in songbirds has been dedicated to the auditory system and the effects of sex steroids on brain function, potential interactions between these two systems have been largely unexplored, particularly at the level of a cortical-like area like NCM. The experiments proposed are therefore a significant departure from previous studies and will open up new avenues of investigation. Our efforts are likely to result in an improved method for directly measuring local estrogen levels in the songbird brain, as well as the potential identification of novel gene targets of estrogen regulation. The results will be highly relevant to understanding how environmental and hormonal factors interact to regulate social and reproductive behaviors in birds and other vertebrates including humans. PUBLIC HEALTH RELEVANCE Although much research in songbirds has been dedicated to the auditory system and the effects of sex steroids on brain function, potential interactions between these two systems have been largely unexplored, particularly at the level of a cortical-like area like NCM. The experiments proposed are therefore a significant departure from previous studies and will open up new avenues of investigation. The results will be highly relevant to understanding how environmental and hormonal factors interact to regulate social and reproductive behaviors in birds and other vertebrates including humans.